Digital printers are devices that are used to produce hardcopy reproductions of digital images. Digital images are digital representation of an image, typically expressed as a rectangular array of pixel values, or as a set of encoded information that can be used to generate such an array of pixel values. Some printers are binary in nature, only being able to produce white and one other output level for any particular pixel. Other printers are capable of producing multiple output levels. Thermal dye transfer printers are one common type of digital printer that produce multiple output levels by varying the amount of heat applied to a thermal dye donor ribbon. Other types of printers would include digital silver halide printers, electrophotographic printers, and ink jet printers.
Color digital printers are used to produce color reproductions of color digital images. Typically color digital printers work by producing multiple color planes using different colorants for each color plane. Commonly, the color planes are cyan, magenta, and yellow (CMY), or cyan, magenta, yellow, and black (CMYK), although many color printers use other special colorants as well.
The images formed by digital printers frequently suffer from a number of artifacts. Artifacts can be defined as undesirable image characteristics. One such class of artifacts is edge artifacts. Edge artifacts occur on the boundaries between various image regions. Although there are a number of kinds of edge artifacts, many of them result when a digital printer produces an undesirable density profile at the edge.
For example, consider FIG. 1 (a) which shows one type of image that is prone to exhibit edge artifacts. The image consists of an inner square 10 of one color, surrounded by an outer square 12 of another color. For the purposes of this example, consider the inner square to be cyan, and the outer square to be magenta. Therefore if this image is printed with a CMY printer the inner square will be printed using the cyan colorant, and the outer square will be printed using the magenta colorant. Frequently when this image is printed, an edge artifact occurs where the boundary between the outer square and the inner is not accurately reproduced. This is shown figuratively in FIG. 1(b) which illustrates an edge artifact 14 in the form of a white ring around the inner square. Depending on the severity of the artifact, the ring may be white, or it may simply be lighter than the surrounding patches.
One cause for the edge artifact shown in FIG. 1, is illustrated in FIGS. 2 (a)-(d). Consider an edge in the image between a cyan region on the left, and a magenta region on the right. FIG. 2(a) shows graphs of the cyan image content 20, and the magenta image content 21, as a function of the image position x. For x positions to the left, there is cyan image content, but no magenta image content. For x positions to the right, the opposite is true. The corresponding cyan pixel values 22 and magenta pixel values 23 used to represent the image content for the cyan and magenta color planes, respectively, are shown in FIG. 2(b). If this image were reproduced on a thermal dye transfer color digital printer, the pixel values will be converted into thermal energy applied to the thermal dye donor ribbon to transfer dye to the receiver layer. Figurative representations of the cyan thermal energy 24 and the magenta thermal energy 25 are shown in FIG. 2(c). It can be seen that the thermal energy profile for the uniform image regions is shown to be somewhat non-uniform, and that some of the thermal energy near the edges has dissipated to the nearby pixels. Also shown on the graphs are the cyan thermal threshold 26 and the magenta thermal threshold 27 corresponding to the thermal energy levels necessary to transfer dye. Energy levels below these thresholds will result in no dye transfer to the receiver layer. The resulting cyan density profile 28 and magenta density profile 29 are shown figuratively in FIG. 2(d). It can be seen that there is some non-uniformity across the uniform image regions, and that the edge profiles are different from the ideal edge profiles shown in FIG. 2(a). The result when the two color planes are superimposed is shown in FIG. 3. At the boundary between the cyan density profile 30, and the magenta density profile 32 there is an edge artifact 34 where the density falls to a much lower value, resulting in a white line similar to that shown in FIG. 1. Other edge artifacts which are closely related to this example include a thinning or a loss of density in narrow lines, and a rounding of comers on objects.